Water and Electrolyte Balance in Animals

1. Water and ElectrolyteBalance in Animals Chapter 42

2. Water Balance and Animals • Chemical reactions of life are in aqueous solutions • An electrolyte is a compound that dissociates into ions when dissolved in water • Cells require precise concentrations of electrolytes—Na+, Cl–, K–, and Ca2+—to function normally • Maintaining water and electrolyte balance is crucial to survival

3. Osmosis and Diffusion • Diffusion- movement of substances from higher concentration to lower concentration along their concentration gradient • Osmosis is the diffusion of water through a selectively permeable membrane from higher water concentration to lower water concentration

5. Osmoregulation

6. Osmoregulation • Animals must maintain a a certain water concentration within tissue • Osmoregulation • Must also deal with metabolic waste • Excretion • Freshwater animals • Show adaptations that reduce water uptake and conserve solutes • Desert and marine animals face desiccating environments • With the potential to quickly deplete the body water

10. Overcoming Osmotic Challenges • Two basic solutions to balancing water gain with water loss • Osmoconformers, which are only marine animals • Are isoosmotic with their surroundings and do not regulate their osmolarity • Only available to marine animals • Osmoregulators expend energy to control water uptake and loss • In a hyperosmotic or hypoosmotic environment

11. Overcoming Osmotic Challenges • Osmoregulation requires energy • Diffusion tries to equalize concentrations in a system • Must expend energy to maintain the osmotic gradients that cause water to move in or out • Use active transport to manipulate solute concentrations in their body fluids • How much energy is dependant on the difference in solute concentration between their body and the outer water

12. Overcoming Osmotic Challenges • Most animals are said to be stenohaline • And cannot tolerate substantial changes in external osmolarity • Euryhaline animals • Can survive large fluctuations in external osmolarity • Salmon, tilapia

13. Marine Animals • Most marine invertebrates are osmoconformers • Differ in concentrations of specific solutes • Most marine vertebrates and some invertebrates are osmoregulators • Marine bony fishes are hypoosmotic to sea water • And lose water by osmosis and gain salt by both diffusion and from food they eat • These fishes balance water loss • By drinking seawater

14. Gain of water and salt ions from food and by drinking seawater Osmotic water loss through gills and other parts of body surface Excretion of salt ions and small amounts of water in scanty urine from kidneys Excretion of salt ions from gills Marine Animals

15. Osmotic water gain through gills and other parts of body surface Uptake of water and some ions in food Uptake of salt ions by gills Excretion of large amounts of water in dilute urine from kidneys Freshwater Animals • Freshwater animals • Constantly take in water from their hypoosmotic environment • Lose salts by diffusion • Excrete large amounts of very dilute urine • Salts lost by diffusion are replaced by foods and uptake across the gills

16. 100 µm 100 µm (b) Dehydrated tardigrade (a) Hydrated tardigrade Animals in Temporary Waters • Some aquatic invertebrates living in temporary ponds • Can lose almost all their body water and survive in a dormant state • This adaptation is called anhydrobiosis

17. Land Animals • Adaptations that reduce water loss are essential to life on land • Some have a body covering: wax, shell, layers of dead skin • Change behavior: nocturnal to reduce water loss in the desert • Still loose a lot of water through feces and urine • Many land animals manage their water budgets by drinking and eating moist foods and by using metabolic water

18. Transport epithelia • Most animals maintain the composition of the cellular cytoplasm by managing the composition of the fluid that bathes the cells • The interstitial fluid • Transport epithelia • Are specialized cells that regulate solute movement • Are essential components of osmotic regulation and metabolic waste disposal • Are arranged into complex tubular networks

19. Nasal salt gland (a) An albatross’s salt glands empty via a duct into thenostrils, and the salty solution either drips off the tip of the beak or is exhaled in a fine mist. Nostril with salt secretions Lumen of secretory tubule Vein Capillary (c) The secretory cells actively transport salt from theblood into the tubules. Blood flows counter to the flow of salt secretion. By maintaining a concentrationgradient of salt in the tubule (aqua), this countercurrentsystem enhances salt transfer from the blood to the lumen of the tubule. Secretory tubule Artery NaCl Transport epithelium (b) One of several thousand secretory tubules in a salt-excreting gland. Each tubule is lined by a transportepithelium surrounded by capillaries, and drains intoa central duct. Direction of salt movement Blood flow Secretory cell of transport epithelium Central duct • An example of transport epithelia is found in the salt glands of marine birds • Which remove excess sodium chloride from the blood

20. Insects • Relatively large surface area with which to lose water and a small volume in which to retain it • Trachea connects with the atmosphere through openings called spiracles • Muscles contract and open the spiracles • Exskeleton made of chitin and layers of protein called cuticle • Layer of wax covers the surface

22. Nitrogenous Wastes

23. Nucleic acids Proteins Nitrogenous bases Amino acids –NH2 Amino groups Many reptiles (including birds), insects, land snails Most aquatic animals, including most bony fishes Mammals, most amphibians, sharks, some bony fishes O H C N C HN C O NH2 C C C O N N O NH3 NH2 H H Ammonia Urea Uric acid Nitrogenous Wastes • The type and quantity of an animal’s waste products • May have a large impact on its water balance • Among the most important wastes • Are the nitrogenous breakdown products of proteins and nucleic acids

24. Forms of Nitrogenous Wastes • Different animals • Excrete nitrogenous wastes in different forms • Vary in their toxicity and energy costs • Animals that excrete nitrogenous wastes as ammonia need access to lots of water • Very toxic and can only be tolerated at low levels • Release it across the whole body surface or through the gills

25. Forms of Nitrogenous Wastes • The liver of mammals and most adult amphibians • Converts ammonia to less toxic urea • Allows for transport and storage in the animal body • Urea is carried to the kidneys, concentrated • And excreted with a minimal loss of water • Main disadvantage is that it takes energy to convert ammonia to urea

26. Forms of Nitrogenous Wastes • Insects, land snails, and many reptiles, including birds • Excrete uric acid as their major nitrogenous waste • Uric acid is largely insoluble in water • And can be secreted as a paste with little water loss • Very high energy cost • Good for those with little access to water

27. Excretory Systems

28. Excretory Systems • Key functions of most excretory systems are • Filtration, pressure-filtering of body fluids producing a filtrate • Reabsorption, reclaiming valuable solutes from the filtrate • Secretion, addition of toxins and other solutes from the body fluids to the filtrate • Excretion, the filtrate leaves the system

29. Capillary Filtration. The excretory tubule collects a filtrate from the blood. Water and solutes are forced by blood pressure across the selectively permeable membranes of a cluster of capillaries and into the excretory tubule. 1 Excretory tubule Filtrate Reabsorption. The transport epithelium reclaims valuable substances from the filtrate and returns them to the body fluids. 2 3 Secretion. Other substances, such as toxins and excess ions, are extracted from body fluids and added to the contents of the excretory tubule. 4 Excretion. The filtrate leaves the system and the body. Urine Excretory Systems • Excretory systems • Regulate solute movement between internal fluids and the external environment • Most excretory systems • Produce urine by refining a filtrate derived from body fluids

30. Types of Excretory Systems • The systems that perform basic excretory functions • Vary widely among animal groups • Are generally built on a complex network of tubules • A protonephridium • Is a network of dead-end tubules lacking internal openings • The tubules branch throughout the body • And the smallest branches are capped by a cellular unit called a flame bulb • These tubules excrete a dilute fluid • And function in osmoregulation

31. Nucleus of cap cell Cilia Interstitial fluid filters through membrane where cap cell and tubule cell interdigitate (interlock) Tubule cell Flame bulb Protonephridia (tubules) Tubule Nephridiopore in body wall Types of Excretory Systems

32. Coelom Capillary network Bladder Collecting tubule Nephridio- pore Metanephridia Nephrostome Types of Excretory Systems • Each segment of an earthworm • Has a pair of open-ended metanephridia • Metanephridia consist of tubules • That collect coelomic fluid and produce dilute urine for excretion

33. Digestive tract Rectum Hindgut Intestine Malpighian tubules Midgut (stomach) Feces and urine Salt, water, and nitrogenous wastes Anus Malpighian tubule Rectum Reabsorption of H2O, ions, and valuable organic molecules HEMOLYMPH Types of Excretory Systems • Metanephridia consist of tubules • That collect coelomic fluid and produce dilute urine for excretion • In insects and other terrestrial arthropods, malpighian tubules • Remove nitrogenous wastes from hemolymph and function in osmoregulation

34. Vertebrate Excretory System • Vertebrates must carefully regulate the osmolarity of their tissues • Depending on conditions, terrestrial animals may need either to conserve or to excrete water and electrolytes to achieve homeostasis • Kidney is the organ responsible for regulating water and electrolyte balance in terrestrial vertebrates

35. The Structure of the Kidney • Occur in pairs and tend to be bean-shaped • Renal artery brings blood containing nitrogenous wastes into the organ • Renal vein carries blood away

36. Renal medulla Renal cortex Renal pelvis Ureter Section of kidney from a rat (b) Kidney structure Mammalian Kidneys • The mammalian kidney has two distinct regions • An outer renal cortex and an inner renal medulla

37. Juxta- medullary nephron Cortical nephron Afferent arteriole from renal artery Glomerulus Bowman’s capsule Renal cortex Proximal tubule Peritubularcapillaries Collecting duct SEM 20 µm Efferent arteriole from glomerulus Distal tubule Renal medulla To renal pelvis Collecting duct Branch of renal vein Descending limb Loop of Henle Ascending limb Vasarecta (d) Filtrate and blood flow (c) Nephron Mammalian Kidneys • The nephron, the functional unit of the vertebrate kidney consists of a single long tubule and a ball of capillaries called the glomerulus

38. Filtration and Reabsorption:The Renal Corpuscle and the Loop of Henle

39. Filtration and Resorption • Secretion and reabsorption in the proximal tubule • Substantially alter the volume and composition of filtrate • Reabsorption of water continues • As the filtrate moves into the descending limb of the loop of Henle • As filtrate travels through the ascending limb of the loop of Henle • Salt diffuses out of the permeable tubule into the interstitial fluid (not permeable to water)

40. Filtration and Resorption • The distal tubule • Plays a key role in regulating the K+ and NaCl concentration of body fluids • The collecting duct • Carries the filtrate through the medulla to the renal pelvis and reabsorbs NaCl • The mammalian kidney • Can produce urine much more concentrated than body fluids, thus conserving water • The mammalian kidney’s ability to conserve water is a key terrestrial adaptation

41. Distal tubule Proximal tubule 4 1 NaCl Nutrients H2O HCO3 H2O K+ NaCl HCO3 H+ K+ H+ NH3 CORTEX Thick segment of ascending limb Descending limb of loop of Henle 2 3 Filtrate H2O Salts (NaCl and others) HCO3– H+ Urea Glucose; amino acids Some drugs NaCl H2O OUTER MEDULLA NaCl Thin segment of ascending limb Collecting duct 3 5 Key Urea NaCl H2O Active transport Passive transport INNER MEDULLA Filtration and Resorption • Filtrate becomes urine as it flows through the mammalian nephron and collecting duct

42. Filtration of Blood • Filtration occurs as blood pressure • Forces fluid from the blood in the glomerulus into the lumen of Bowman’s capsule • Filtration of small molecules is nonselective • And the filtrate in Bowman’s capsule is a mixture that mirrors the concentration of various solutes in the blood plasma • From Bowman’s capsule, the filtrate passes through three regions of the nephron • The proximal tubule, the loop of Henle, and the distal tubule • Fluid from several nephrons • Flows into a collecting duct

43. Filtration: The Renal Corpuscle • The renal corpuscle is composed of the glomerulus and Bowman’s capsule • Closed end of the nephron forms a capsule that encloses a cluster of capillaries (the glomerulus) that bring blood to the nephron from the renal artery

44. Glomerulus • Glomerular capillaries have large pores. These blood vessels are also surrounded cells whose membranes fold into a series of slits and ridges • Pressure is higher inside the glomerulus than it is in the surrounding capsule. • This pressure differential forces water and solutes out of the blood and into the capsule space, resulting in the formation of a pre-urine.

46. Reabsorption:The Proximal Tubule • Fluid leaves the Bowman’s capsule and enters the proximal tubule The fluid entering the proximal tubule contains water, waste products, and valuable nutrients.

47. Resorption • Na+/K+-ATPase in the basolateral membranes removes intracellular Na+ and creates a gradient for Na+ entry • Takes a lot of energy • In the apical membrane, Na+-dependent cotransporters simultaneously bind Na+ and another solute such as glucose, an amino acid, or Cl– • Water follows the movement of these solutes via osmosis

49. Solute Gradients and Water Conservation • The production of hyperosmotic urine is possible only because a large amount of energy is consumed • Active transport to transport solutes against the concentration gradient • Nephrons produce a region of high osmolarity in the kidney which can be used to extract water from the filtrate in the collecting duct • Two solutes, NaCl and urea, contribute to the osmolarity of the interstitial fluid • Which causes the reabsorption of water in the kidney and concentrates the urine

50. The Loop of Henle • Arrangement of parallel tubes • Carry fluids in opposite directions to transfer soluble substance between the tubes • Sets up an osmotic gradient in which an exchange of water and solutes occurs between each of the segments in the loop and the cells outside